diff --git a/lib/context.js b/lib/context.js
index 5bb5107..88ec1ae 100644
--- a/lib/context.js
+++ b/lib/context.js
@@ -126,6 +126,7 @@ const proto = module.exports = {
     const { res } = this;
 
     // first unset all headers
+    /* istanbul ignore else */
     if (typeof res.getHeaderNames === 'function') {
       res.getHeaderNames().forEach(name => res.removeHeader(name));
     } else {
@@ -159,6 +160,8 @@ const proto = module.exports = {
  * @return {Object}
  * @api public
  */
+
+/* istanbul ignore else */
 if (util.inspect.custom) {
   module.exports[util.inspect.custom] = module.exports.inspect;
 }
diff --git a/test/context/onerror.js b/test/context/onerror.js
index ddee81c..f1a44a0 100644
--- a/test/context/onerror.js
+++ b/test/context/onerror.js
@@ -129,7 +129,26 @@ describe('ctx.onerror(err)', () => {
           .expect('Internal Server Error');
       });
     });
+    describe('when ENOENT error', () => {
+      it('should respond 404', () => {
+        const app = new Koa();
 
+        app.use((ctx, next) => {
+          ctx.body = 'something else';
+          const err = new Error('test for ENOENT');
+          err.code = 'ENOENT';
+          throw err;
+        });
+
+        const server = app.listen();
+
+        return request(server)
+          .get('/')
+          .expect(404)
+          .expect('Content-Type', 'text/plain; charset=utf-8')
+          .expect('Not Found');
+      });
+    });
     describe('not http status code', () => {
       it('should respond 500', () => {
         const app = new Koa();